Several methods and devices have traditionally been used to measure the water or moisture content of bulk materials, such as soils and food products. One well-known technique is to measure changes in the dielectric constant of the medium being measured. The dielectric constant of water is approximately 80, the dielectric constant of soil minerals and organic matter is around 4, and the dielectric constant of air is 1. Accordingly, changes in water content of a particular medium will result in large changes in the dielectric constant of the medium, which can readily be measured.
There are numerous specific examples where knowing the moisture or water content can be critical. For example, without limitation, the moisture content of soil can give information which is useful for conserving applied irrigation water or reducing ground water contamination. Moisture content measurements on stored grain can be used to prevent spoilage.
A particular problem with measuring and monitoring moisture content of materials, particularly soils, has been the expense and sophisticated nature of the equipment used to measure or monitor the soil. Traditional devices for measuring moisture content in soils have been relatively large and very expensive to manufacture. Generally, many hundreds of dollars have traditionally been required to manufacture a single moisture measuring apparatus. This has traditionally made it cost prohibitive for those in agriculture, for example, to use multiple moisture content measuring devices in a field.
An explanation of use of a transmission line buried in soil to detect the presence of moisture in the soil by measuring the travel time of an electrical pulse in the transmission line is found in the publication entitled Evaluation of simple Transmission Line Oscillators for Soil Moisture Measurement, 20 COMPUTERS AND ELECTRONICS IN AGRICULTURE (1998), pp. 31-44, authored by Gaylon S. Campbell and Russell Y. Anderson, which is incorporated in its entirety by this reference.
Various other methods and apparatus exist for detecting the presence of moisture in porous materials. For example, many devices, such as that shown in U.S. Pat. No. 5,148,125, determine the presence of moisture in a material by measuring the propagation delay of an AC signal applied to a transmission line buried in the material. Systems such as these suffer from the disadvantage that, for operation, a user must have access to both ends of the transmission line.
In view of the foregoing, there is a need to provide a moisture detection apparatus and method which will be relatively inexpensive to manufacture, easy to use in the field, and provide accurate data concerning the moisture content of porous materials.